This invention relates to an exposure technique used in pattern transfer in a semiconductor process and, in particular, to a reflective-type mask blank for exposure, a method of producing the same, and a reflective-type mask for exposure.
In the semiconductor industry, a pattern transfer technique capable of transferring much finer patterns is required following the development of a semiconductor device more and more miniaturized. As the pattern transfer technique of the type, attention is recently directed to an EUV (Extreme Ultra Violet) lithography which is an exposure technique using EUV light having a shorter wavelength. It is noted here that the EUV light is a radiation having a wavelength within a soft X-ray region or a vacuum ultraviolet region, specifically, a wavelength within a range between about 0.2 and 100 nm.
As a mask used in the EUV lithography, proposal is made of a reflective-type mask disclosed, for example, in Japanese Unexamined Patent Publications Nos. H08-213303 (JP 8-213303 A) and H07-333829 (JP 7-333829 A). Referring to FIG. 1, the reflective-type mask comprises a substrate 1, a reflective multilayer film 2 formed on the substrate 1 for reflecting light, an intermediate layer pattern 3 formed on the reflective multilayer film 2, and an absorber pattern 4 formed on the intermediate layer pattern 3 for absorbing light.
In an exposure apparatus, light incident to the reflective-type mask is partially absorbed in an area where the absorber pattern 4 is present and is partially reflected by the reflective multilayer film 2 in a remaining area where the absorber pattern 4 is not present. The former area and the latter area may be referred to as an absorption region and a reflection region, respectively. A reflected image formed by the light reflected by the reflective multilayer film 2 is transferred through a reflection optical system onto a wafer. It is noted here that the intermediate layer pattern 3 of the mask used as described above is not yet formed but an intermediate layer without any pattern covers and protects the reflective multilayer film when the absorber pattern 4 is formed by dry etching or the like in a mask production process Thereafter, the intermediate layer is etched in conformity with the absorber pattern 4 to produce the intermediate layer pattern 3 illustrated in FIG. 1.
As the reflective multilayer film 2, use is made of a structure in which a plurality of kinds of materials different in refractive index for the wavelength of exposure light are periodically laminated by the thickness on the order of nanometers. In case where X-ray is used as a light source, the reflective multilayer film generally has a periodic multilayer structure comprising heavy elements such as Mo, Ru, and W and light elements such as Si and C. For the exposure light having a wavelength of, for example, about 13 nm, use may be made of the reflective multilayer film comprising a lamination of about 40 periods of Si films of 4 nm thick and Mo films of 3 nm thick.
Among the materials forming the reflective multilayer film, the heavy elements are susceptible to time variation in characteristics as a result of oxidization or the like. Taking this into account, a topmost layer of the reflective multilayer film generally comprises a light element such as Si. Japanese Unexamined Patent Publication No. H08-293450 (JP 8-293450 A) discloses a reflective-type mask addressed to the problem of the time variation in case where the heavy element is used as the topmost layer. In the reflective-type mask disclosed in the above-mentioned publication, an upper surface of the reflective multilayer film and the absorber film are covered with a coating layer made of a material small in time variation of optical constants.
However, the present inventors have found out that, even if the topmost layer of the reflective multilayer film comprises a light element such as Si or even if the heavy element is prevented from being exposed on the upper surface of the reflective multilayer film, the reflective multilayer film yet exhibits the time variation. Such time variation of the reflective multilayer film is a factor causing the decrease in reflectivity and in positional accuracy of the mask.
Furthermore, the present inventors found out that the time variation of the reflective multilayer film results from a damage through a side surface of the reflective multilayer film. Specifically, a peripheral end of a very thin layer of the heavy element is exposed at the side surface of the reflective multilayer film. The damage through the peripheral end causes variation in stress of the reflective multilayer film and time variation such as decrease in reflectivity.
It is therefore an object of this invention to prevent time variation of a reflective multilayer film.
It is another object of this invention to provide a reflective-type mask improved in reliability without deterioration in pattern transfer accuracy by preventing time variation of a reflective multilayer film.
It is still another object of this invention to provide a reflective-type mask blank for use in producing a reflective-type mask of the type.
As described above, the present inventors have found out that the time variation of the reflective multilayer film is caused also by the damage through the side surface thereof and that, by providing a protection layer on the side surface of the reflective multilayer film, the above-mentioned objects are achieved.
According to a first aspect of this invention, there is provided a reflective-type mask blank for exposure, comprising a substrate, a reflective multilayer film formed on the substrate for reflecting exposure light, and an absorber film formed on the reflective multilayer film for absorbing the exposure light, the reflective multilayer film comprising heavy element material films and light element material films different in refractive index from each other and alternately laminated, wherein:
the mask blank further comprises a protection layer for protecting peripheral ends of at least the heavy element material films in the reflective multilayer film.
According to a second aspect of this invention, there is provided a reflective-type mask blank for exposure according to the first aspect, wherein the protection layer comprises a thin film selected from a group of thin films same as the light element material films of the reflective multilayer film formed on the substrate and the absorber film formed on the reflective multilayer film and covers at least the peripheral ends of the heavy element material films.
According to a third aspect of this invention, there is provided a reflective-type mask blank for exposure according to the first aspect, wherein the protection layer is made of a material containing Si.
According to a fourth aspect of this invention, there is provided a reflective-type mask for exposure comprising a reflective-type mask blank according to any one of the first through the third aspects and a pattern formed on the absorber film of the reflective-type mask blank.
According to a fifth aspect of this invention, there is provided a method of producing a reflective-type mask blank for exposure, comprising the steps of forming on a substrate a reflective multilayer film which comprises heavy element material films and light element material films different in refractive index from each other and alternately laminated and which is adapted to reflect exposure light having a specific wavelength, and forming on the reflective multilayer film at least an absorber film for absorbing the exposure light, wherein:
at least one of a plurality of deposition steps each of which follows the deposition of each of the heavy element material films and deposits each of other films of the reflective multilayer film except the heavy element material films is selected as a selected deposition step;
a film material of at least one of the other films is deposited in the selected deposition step over a deposition area greater than a plan-view deposition area of each heavy element material film so that at least a peripheral portion of each heavy element material film is substantially entirely covered.